1. Field of the Invention
This invention relates to a method of producing a hydrophilic membrane by grafting acrylic acid and/or methacrylic acid onto an irradiated polyethylene base film. The method is characterized by exposing a polyethylene film having a thickness of 150 .mu.m or lower to ionizing radiation in air to thereby increase greatly the rate of subsequent grafting and provide a hydrophilic membrane having very low electrical resistance.
2. Description of the Prior Art
Grafting onto a polyethylene base film that has been irradiated in vacuum or a nitrogen atmosphere is generally considered to initiate at radicals trapped in the crystalline region of the polyethylene. When such irradiation is effected in air or an oxygen atmosphere, radicals formed by radiation react with oxygen to form hydroperoxide (ROOH) or diperoxide (ROOR), and grafting is considered to initiate at radicals produced by the thermal decomposition of such hydroperoxide and diperoxide. These peroxides are known to be relatively stable and withstand extended storage at room temperature. Grafting to a polyethylene film irradiated in air or an oxygen atmosphere generally requires higher temperature (at least 60.degree. C.) than with a polyethylene film irradiated in an nitrogen atmosphere. It is also known that oxygen does not easily diffuse into the crystalline region of polyethylene and that because of slow diffusion of oxygen into the center region of a thick polyethylene film, the effect of oxygen is not conspicuous in grafting to a polyethylene film that has been irradiated with a high dose rate of radiation such as electron beams.
As a result of studies on grafting to various types of irradiated polymer film, the inventors of this invention have found that grafting to a thin polyethylene film can be carried out at a very high rate without particularly increasing the reaction temperature even if the film has been subjected to irradiation in air and that the resulting membrane of graft polymer has very low electrical resistance.